Offshore drilling rigs normally employ a riser to connect the subsea wellhead with the drilling rig. A blowout preventer is located at a lower end of the riser. Land rigs also use blowout preventers. A blowout preventer is a large assembly having many features for closing the riser in the event high pressure in the wellbore begins pushing the drilling mud upward. Those features include an annular blowout preventer that seals around pipe regardless of the diameter. The blowout preventer also has pipe rams that are sized to close and seal around pipe strings of certain diameters. The blowout preventer also has shear rams that will shear a drill pipe string or a production tubing string in the event of an emergency.
Pipe shear rams have two rams, each of which has a blade mounted to it. Pistons move the rams toward each other to shear pipe extending through the blowout preventer. One blade is located at a higher elevation than the other and slides over the lower one when the shear rams close. Often the pipe string will be off-centered relative to the axis of the bore extending through the shear ram assembly. The blades have converging side portions that lead to a central area for centering the pipe as the rams close. In some pipe ram assemblies, a remote possibility exists that the pipe string will remain at an outside wall of the bore during closure, which could lead to incomplete shearing of the pipe string.